In recent years, by the miniaturization of a semiconductor manufacturing process and higher integration thereof, an SoC (System-on-a-Chip) in which the almost entire systems are integrated on one LSI (Large Scale Integration) has been generalized. When compared with the case in which a plurality of single-function LSIs are implemented on a board, the SoC brings about various advantages such as reduction of occupation area on the print board, speeding up, reduction of electric power consumption, and reduction of cost.
Since the SoC realizes an analog circuit on a chip, the SoC has a problem different from that of a simple LSI. For example, when a plurality of independent power source area are provided inside one chip, and each of the electric power source is independently controlled to be blocked, there is a problem for the signal wiring in the chip. Accordingly, a semiconductor device is provided, configured by including, as a unit to observe the voltage changes inside the LSI, a voltage monitor which converts voltage changes into frequency changes by a ring oscillator configured by a MISFET (Metal-Insulator-Semiconductor Field-Effect Transistor) of a first gate insulation film thickness, similar to a MISFET configuring the logic circuit; and an amplification circuit which outputs the output signal to outside of the LSI, and the voltage monitor is configured by a MISFET having small threshold value so as to operate even when the electric power source voltage has a lower value (Patent Literature 1).